Radio antenna proximities

Submitted: Friday, Oct 25, 2002 at 00:00
ThreadID: 2218 Views:1370 Replies:9 FollowUps:7
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Is there a limit to how close I can run a 27mHz and UHF antenna on my bullbar where the closeness would affect transmission/reception of the radios? I want to mount both antennas on the LHS of the bar about 300mm apart.

thanks
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Reply By: Truckster - Friday, Oct 25, 2002 at 00:00

Friday, Oct 25, 2002 at 00:00
I had mine side by side on the LHS, around 30cm apart, like 100's of others, but I was having problems...


I went to Telstat Communications, the local radio place, and they said your better off with one on each side of the bar.. Or as far apart as possible.

I think it depends on the antenna's themselves, power etc.. Im not sure, but call a reputable radio place would be your safest bet.
AnswerID: 7844

Follow Up By: Bonza - Friday, Oct 25, 2002 at 00:00

Friday, Oct 25, 2002 at 00:00
Hmmmmm that was my main worry, What problems were you having?
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FollowupID: 3636

Reply By: Truckster - Friday, Oct 25, 2002 at 00:00

Friday, Oct 25, 2002 at 00:00
I could sit next to a mate with the same radio, and ariel, and he could talk to people, here then, all I would hear is Bernie saying to me what did I think of that idea, I would have no idea what he was talking about!!! I wouldnt even get crackling or anything from the other party!!
Also apart from no range, the guy said one would interfere with the other in other ways, he explained it but it was in techo-chubby terms..

But I have seen LOTS of otehr cars with ariels close together, maybe it was just Uniden Radios, but I couldnt see it being that.

Since I dont have my ariels on 24/7 the one on the left (27meg) only goes on when Im in the bush.
AnswerID: 7846

Reply By: voxson - Friday, Oct 25, 2002 at 00:00

Friday, Oct 25, 2002 at 00:00
Hi. I was told by a communications expert to mount the 27meg antenna on the bullbar and the uhf on the gutter, which i did and dont have any problems. He said to mount the uhf as high as possible because it is height dependant.
AnswerID: 7849

Follow Up By: Truckster - Friday, Oct 25, 2002 at 00:00

Friday, Oct 25, 2002 at 00:00
Center of the roof is the ideal place, roofrack or drill a hole in the roof!



But not all of us wanna drill holes in the roof!
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FollowupID: 3637

Reply By: Ranga - Friday, Oct 25, 2002 at 00:00

Friday, Oct 25, 2002 at 00:00
Bonza,

I used the rear ladder on the 2002 Defender to mount the 27(meg) aerial. Seems to work OK. Do you have a similar option.

Tom
AnswerID: 7854

Follow Up By: Bonza - Friday, Oct 25, 2002 at 00:00

Friday, Oct 25, 2002 at 00:00
Tom, I opted for a comfortable vehicle not an agricultural piece if equipment. I dont have a rear ladder on my 2001 GU Patrol, but you dont have a roofrack so where does the ladder go to ???????? Just the antenna?
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FollowupID: 3638

Reply By: Ranga - Friday, Oct 25, 2002 at 00:00

Friday, Oct 25, 2002 at 00:00
Ah yes you are correct! But it can carry the weight of an extra aerial with no problems.
AnswerID: 7855

Reply By: Eric - Friday, Oct 25, 2002 at 00:00

Friday, Oct 25, 2002 at 00:00
Bonza.
Let me try and explain with out using techno chubby terms.
The receiver in your radio is very sensitive so that it can pick up a weak signal. The other radio althow it is operating on a different frequency will still force its way into the reciever and cause it to be overloaded. The best place for your uhf is definately the roof centre butt the other good place is off the spare wheel mount, a lenght of 32 mm square tube bolted to the wheel brachet with a 75mm round disk welded onthe top to mount the antenna works well in all directions.Run the coax thru the rear wiper loom.
Eric
AnswerID: 7865

Follow Up By: Truckster - Wednesday, Oct 30, 2002 at 01:00

Wednesday, Oct 30, 2002 at 01:00
Eric, you seem to be the man, is there a limit to how long the cables can be?

Eg If I ran the UHF off the rear wheel carrier, or the 27meg, would the length of the cable from radio to ariel affect reception or transmission?

Thanks
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FollowupID: 3759

Follow Up By: Bonza - Wednesday, Oct 30, 2002 at 01:00

Wednesday, Oct 30, 2002 at 01:00
Turckster,

I spoke to a friend in the radio game and he tells me that the cable length on the UHF is not a problem, even from the top of a ladder (with or without roofrack) at the rear of the vehicle
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FollowupID: 3775

Follow Up By: Eric - Wednesday, Oct 30, 2002 at 01:00

Wednesday, Oct 30, 2002 at 01:00
Truchster.
The length of cable is best short but the signal will not be weakened much by 1or2 metres, the quality of cable and the height are more important. The thing that most effects the satisfactory use of the radio is what is called the signal to niose ratio. the reason is that if you place the antenna near the motor the noise picked up will cause you to turn up the sqealch and therefore not hear distant calls that is why a small antenna mounted on the roof is better than some high gain pole mounted on the bull bar.
Eric
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FollowupID: 3785

Reply By: dave - Saturday, Oct 26, 2002 at 00:00

Saturday, Oct 26, 2002 at 00:00
i've been asked this question a few times. I have 4 aerials on the LHS of my bullbar.
27meg/UHF GSM/CDMA.

Now the following is granted, both of the phones transmit a small packet of their own accord, there may be a small interuption during these, second or so long bursts.

BUT, your UHF and 27meg aren't transmitting if you are not using them, so unless you are trying to use both radios at the same time, (how often is that neccesary, really) there should be no interferance from the other unit.

Remember that we are surrounded by all sorts of invisible radio waves all day everywhere, it's not like our aerials attract them, rather they happen to stumble accross them.

I accept that one areial may place a 'shadow' over the one next to it. But even the thickest aerial is little over an inch.

For me, the quality of the aerial, cable, and the job you did routing the cable and fixing the connectors, the quality of the power cable and how directly it gets its power, are the real determinants of the signal quality you will recieve.

And you're right, maybe it's because you are using a uniden.

my 2c worth.

Dave
AnswerID: 7869

Follow Up By: Truckster - Saturday, Oct 26, 2002 at 00:00

Saturday, Oct 26, 2002 at 00:00
Was usin Uniden! Never again...
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Reply By: Darryn - Saturday, Oct 26, 2002 at 00:00

Saturday, Oct 26, 2002 at 00:00
Bonzer and others, always mount antennas as far appart as possible. 27mhz antennas are dependant on length and correct mounting to work properly. Having any other type of antenna near by will effectivly lengthen it and alter the SWR which leads to a reduction in transmitted power on 27mhz. As you only have 4 watts on AM and 12 watts on SSB this causes severe reductions in effective range and could even cause damage to the set if the SWR is raised to far. UHF isn't bothered as much but anything that can reflect radio waves will cause a "shadow" in the radiation pattern on that side. Despite what is said by manufacturers I still check any 27mhz installation with an SWR meter to ensure correct installation and performance.
Regd's Darryn
AnswerID: 7873

Reply By: Member - Nigel - Monday, Oct 28, 2002 at 01:00

Monday, Oct 28, 2002 at 01:00
Something that hasn't been mentioned is that having antennas close together can "pull" the signal from one antenna towards the other, thereby reducing range in all other directions. The opposite is also possible (depending on the exact configuration): the signal can be pushed in the opposite direction to the adjacent antenna.

To avoid this it's best to have all antennas at least a half wave lenght away from each other.

Half wave length for UHF is around 34 cm
Half wave length for 27 MHz is approx 5.5 metres - not really possible, but the more distance the better.
AnswerID: 7967

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